Hydrostatic machine

ABSTRACT

The valve plate of a reversible hydrostatic axial or radial piston machine surrounds an eccentric of the shaft and comprises two rings which are sealingly telescoped into each other and are biased apart so that each thereof engages an inner face of the housing. A first chamber of the housing which surrounds the valve plate communicates with one of the ports and a second chamber, defined by the valve plate, communicates with the other port. The valve plate alternately connects the fluid pressurizing spaces of the machine with the first and second chambers when the eccentric rotates with the shaft or when the housing rotates with reference to the eccentric.

United States Patent Inventor Walter Lubos [56] References Cited Sprockhvel Westphalia, Germany UNITED STATES PATENTS QIIP' No' s'gg 2,969,810 1/1961 Dudley 137/625.15 Pgfemed Jucn; 15 1971 3,034,484 3/1962 srefancin. 12s/12 X Assi nee G Dues;erloh G m b H 3,174,403 3/1965 McCoy.... 251/281 X g Srockhoevel/wstl'm'a'a Germany 3,316,814 5/1967 Charlson 137/625.21X Priority Oct. s, 1968 3,370,611 2/1968 Flint 251/175X Germany Primary Examiner- Henry T. Klinksiek P180154L3 Attorney-Michael S. Striker ABSTRACT: The valve plate of a reversible hydrostatic axial or radial piston machine surrounds an eccentric of the shaft and comprises two rings which are sealingly telescoped into loykps'rglc MAPmE each other and are biased apart so that each thereof engages alms rawmg lgs' an inner face of the housing. A rst chamber of the housing U.S. Cl ..l37/625.21, which surrounds the valve plate communicates with one ofthe 418/61 ports and a second chamber, dened by the valve plate, comllnt. Cl .Flk ll/02, municates with the other port. The valve plate alternately con- 1`04c 17/02 nects the fluid pressurizing spaces of the machine with the first Field of Search 137/625.21 and second chambers when the eccentric rotates with the 625.24; 251/175, 185, 281, 283; 123/8, l2; 91/56; shaft or when the housing rotates with reference to the eccen- 103/130; 230/145; 418/61 tric.

| 1 l f /u/ 1 /V /ii /2 9.0 l l 7/ I l 1 PATENIED Juan 5 mi PATENTED Juul 5 Ism SHEET 2 0F 2 HYDROSTATIC MACHINE BACKGROUND OF THE INVENTION The present invention relates to hydrostatic machines in general, particularly to hydraulically operated radial or axial piston pumps or motors. Still more particularly, the invention relates to improvements in pumps or motors of the type wherein the shaft or housing is preferably rotatable in two directions so that the suction port constitutes the discharge port and vice versa when the direction of rotation is changed. The invention relates mainly to improvements in control members, known as valve plates, which are used in such machines to regulate the flow of fluid from the suction port into the pressurizing space or spaces and from the pressurizing space or spaces to the outlet port.

In certain presently known hydrostatic machines wherein the valve plate is located between two parallel faces of the housing and rotates about its axis with reference to the housing, or vice versa, the wear on the end faces of the valve plate increases in a direction from the axis toward the periphery because the peripheral speed increases in the same direction. With time, the end faces of the valve plate assume a convex shape and the adjoining faces of the housing assume a complementary concave shape. Such deformation of originally f'lat faces causes leakage which reduces the output of the machine, especially when the machine is used to effect substantial pressurization of a hydraulic fluid. Attempts to reduce the just described undesirable wear on the faces of the valve plate and on the adjoining faces of the housing include the utilization of relatively large valve plates wherein the fluid flow regulating openings are located at a greater distance from each other. Such proposals have met with little success because it is quite difficult to insure proper lubrication of relatively large surfaces and also because such overdimensioning of the valve plate unduly increases the volume and weight of the machine. A drawback common to all of the aforedescribed conventional machines is that it is difficult to insure proper engagement between four faces, namely, between the two faces of the valve plate and the adjoining faces of the housing. This can be achieved only with substantial expenditures in time and by resorting to complicated precision finishing machinery.

In hydrostatic machines wherein the valve plate is pressed against the control face of the housing by hydraulic fluid and wherein each revolution of the rotary part causes a complete forward and return stroke of rotary or axial pistons, one end face of the valve plate is subjected to pressure of hydraulic fluid along a circle whose center is located on the axis of rotation. The reaction field in the valve plate is eccentric due to the provision of fluid-conveying channels. Consequently, at least some overdimensioning of the valve plate is desirable and normally necessary in order to insure that the eccentric reaction forces cannot tilt the valve plate; such tilting would permit direct flow of fluid between the inlet port and the outlet port. Overdimensioning of the valve plate produces excessive friction, increased wear and a reduction in output. Furthermore, a hydraulically held valve plate acts not unlike a disc brake which is very undesirable in hydraulic motors with a high rate of fluid throughput as well as in motors which produce relatively little torque. The mass of revolving parts is often excessive so that the time required for acceleration and deceleration of such parts is too long.

SUMMARY OF THE INVENTION An object of the invention is to provide a hydrostatic machine, particularly a hydraulic axial or radial piston pump or motor, wherein the valve plate is constructed, assembled and mounted in a novel and improved way.

Another object of the invention is to provide a hydrostatic machine wherein the wear on the entire valve plate is uniform and wherein such wear is less pronounced than in presently known machines.

A further object of the invention is to provide a hydrostatic machine wherein the valve plate is in proper sealing engagement with adjoining parts even if it is subjected to lesser mechanical and/or hydraulic stresses than in presently known machines, and wherein the sealing action between the valve plate and the adjoining parts is not affected by wear on the faces which move with reference to each other.

A concomitant object of the invention is to provide a hydrostatic machine wherein the valve plate can be placed into requisite sealing engagement with the adjoining parts of the machine even if the end faces of the valve plate and/or the complementary faces of the adjoining parts are not exactly parallel to each other.

Still another object of the invention is to provide a hydrostatic machine wherein the high-pressure and low-pressure chambers are sealed from each other in a novel and improved way, and wherein such sealing is achieved without adding to the bulk and/or weight ofthe machine.

The invention is embodied in a hydrostatic machine which may constitute an axial or radial piston motor or pump and comprises first and second units one of which is rotatable with reference to the other unit in at least one direction, preferably in two directions. The first unit includes a housing defining a first annular chamber and having a pair of parallel internal faces flanking the ends of the chamber, and the second unit includes a shaft mounted in the housing and an eccentric provided on the shaft in the chamber. The machine further comprises a novel annular control member or valve plate which rotatably surrounds the eccentric in the first chamber and defines a second annular chamber which is sealed from the first chamber. The control member comprises a pair of rings movable axially of each other and each having an annular end face, and means for biasing the rings axially apart so that each end face bears against a face of the housing. The other unit (i.e., the one which does not rotate) is provided with fluid-admitting and discharging ports each communicating with one of the chambers and the housing is formed with at least one fluidpressurizing space terminating in one face of the housing and alternately communicating with the first and second chambers in response to relative movement between the housing and the control member on rotation of the one unit.

The means for biasing the rings of the control member axially apart may include mechanical and/or fluid-operated biasing means. ln accordance with a feature or the invention, all points of the end faces on the rings of the control member perform substantially translatory movements relative to the corresponding faces of the housing in response to rotation of the one unit, and the extent of such translatory movement equals the radius of eccentricity of the eccentric. One of the two rings is preferably sealingly telescoped into the other ring and friction-reducing means (e.g., a needle bearing or a ball or roller bearing) is preferably interposed between the eccentric and the adjoining ring of the control member.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved hydrostatic machine itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary axial sectional view of a rotary piston machine which embodies one form of the invention and wherein the eccentric is integral with a rotary shaft;

FIG. 2 is a similar fragmentary axial sectional view of a second rotary piston machine wherein the eccentric is movable axially of and rotates with the shaft;

FIG. 3 is a fragmentary axial sectional view of a third machine constituting a modification of the machine shown in FIG. 2; and

FIG. 4l is a fragmentary axial sectional view of a fourth machine wherein the housing rotates with reference to the eccentric.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a portion of a radial piston machine having a housing including an end wall or cover 1 provided with ports 2, 3 one of which is the inlet or suction port and the other of which is the outlet or discharge port, depending on the direction of rotation of the shaft 6. The housing further includes a cylindrical wall or shell l which is sealingly connected with the cover ll. The shaft 6 has an integral eccentric end portion which moves a valve plate, here shown as a ring-shaped control disc 7, by way of a needle bearing S. The periphery of the control disc 7 is surrounded by a ring-shaped chamber 26 of the cover 1. The control disc comprises a substantially ringshaped inner portion 9 (hereinafter called inner ring) and a ring-shaped outer portion 10 (hereinafter called outer ring). The inner ring is sealingly telescoped into the outer ring in such a way that the two rings are movable axially relative to each other. The outer ring 10 has an annular end face Il which bears against the adjacent inner face 12 of the cover 1. An annular end face 13 of the inner ring 9 bears against the adjacent outer face or control face 141 of the shell 4. The end face 13 is formed with an endless circular groove or chamber 15 and the inner ring 9 is further provided with a second endless circular groove 16 which communicates with the groove or chamber 15 by way of circumferentially spaced axially parallel apertures or bores 17.

An elastically deformable ring-shaped sealing element 18 is interposed between the rings 9, 10 to prevent leakage of fluid between the groove 16 and the chamber 26. The sealing element 18 also serves as a means for biasing the rings 9, 10 axially and away from each other.

The inner ring 9 is provided with two ring-shaped surfaces 20, 21 located opposite similar ring-shaped surfaces 22, 23 of the outer ring 10. The surfaces 2li-23 are located in planes extending at right angles to the axis of the shaft 6. Fluid which exerts pressure against the surfaces -23 tends to bias the rings 9, 10 axially and away from each other. The shell 4 of the housing is provided with axially parallel fluid-pressurizing spaces or channels 25 having end portions 24 machined into the end face 14. When the control disc '7 moves in the chamber 26 in response to rotation of the shaft 6 and its eccentric 5, it causes the end portions 24 of the spaces 25 to communicate alternately with the groove or chamber l5 and chamber 26. The cover 1 is provided with a compartment 19 communicating with the groove 16 and port 2; this compartment is located substantially behind the control disc 7. The manner in which the radial piston or pistons of the machine shown in FIG. 1 are moved in response to rotation of the shaft 6 is well known in the art and forms no part of the present invention.

FIG. 2 illustrates a portion of a second radial piston machine. All such parts of this machine which are clearly analogous to the corresponding parts of the machine shown in FIG. l are denoted by similar reference characters. The onepiece sealing element 18 of FIG. l is replaced by a ring-shaped sealing element 28 which is recessed into the inner ring 9 and performs only a sealing function, and a package of dished springs 27 interposed between the surfaces 20, 22 and tending to move the rings 9, 10 axially and away from each other. The eccentric 5 of FIG. 2 rotates with but is separably mounted on a polygonal end portion or stub 29 of the shaft 6. The stub 29 permits axial movements of the eccentric 5 with reference to the shaft 6. A helical spring 30 reacts against a ring-shaped retainer 31 mounted on a cylindrical extension 32 of the stub 29 and bears against an internal shoulder of the eccentric 5 to bias the inner end face 33 of the eccentric against the outer face 14 of the shell 4. A cap 34 which is threaded onto the eccentric 5 extends into the compartment 19 and acts not unlike a plunger to assist the spring 30 in maintaining the faces 33, 14

in sealing engagement with each other. A suitable seal is interposed between the cap 34 and eccentric 5 to seal the compartment 19 from the space surrounding the extension 32. Fluid filling the compartment 19 exerts pressure against the end face of the cap 34 to thereby urge the end face 33 against the outer face 14.

The inner ring 9 of the control disc 7 in the machine of FIG. 2 constitutes the outer race of an antifriction bearing which further includes an annulus of circumferentially spaced rolling elements in the form of balls 35 received in a cage 36 and running in a circumferential groove of the eccentric 5. Thus, the latter constitutes the inner race of the antifriction bearing. The gaps between the balls 35 constitute apertures performing the function of bores 17 shown in FIG. 1. The groove 16 of FIG. l is replaced by the space 16a within the outer ring 10. When the shaft 6 rotates with the eccentric 5, the control disc 7 moves in the chamber 26 of the cover 1 and enables the piston means of the machine to draw fluid from the port 2 through the compartment 19, space 16a, apertures between the balls 35, groove or chamber 15a and into the end portions 24 of spaces 25. In response to continuing rotation of the shaft 6, the piston means expels the thus introduced fluid by way of the chamber 26 and port 3. The direction of fluid flow is reversed if the direction of rotation of the shaft 6 is changed. The operation ofthe machine shown in FIG. 1 is analogous.

When the shaft 6 rotates the eccentric 5, all points on the faces 11 and 13 of the control member 7 perform a substan tially translatory movement relative to the faces 12 and 14 of the housing. The extent of such translatory movement equals the radius of eccentricity of the eccentric 5. During each revolution of the shaft 6, each fluid-pressurizing space 25 communicates once with the outer chamber 26 (i.e., with the port 3) and once with the inner chamber 15 or 15a (i.e., with the port 2). Since all points on the end faces 11 and 13 cover equal distances when the shaft 6 rotates, the wear on such end faces and on the adjoining faces 12, 14 is uniform so that all of these faces remain flat even after prolonged use of the machine. The wear is less than in conventional machines because the extent of relative movement is less than in machines where the valve plate performs exclusively a rotary movement.

All of the faces 11-14 are lubricated either by the fluid in the chamber 26 or by lubricant in the lubricating groove so that the lubrication is simpler and more thorough than in known machines. This also results in reduced wear on the control member and housing.

The feature that the control member 7 comprises two rings which are biased axially apart insures that the end faces 1 1, 13 are in requisite sealing engagement with the faces 12, 14 even if such faces undergo considerable wear and even if the faces 12, 14 are not exactly parallel to each other. The mechanical biasing action of the sealing element 18 or of the springs 27 is assisted by the fluid which exerts pressure against the surfaces 20, 22 or 21, 23, depending on the direction of rotation of the shaft 6, i.e., depending upon whether the pressurized fluid fills the chamber 15 (or 15a) or the chamber 26. It was found that the combined mechanical and hydraulic biasing means which tend to move the rings 9, l0 axially apart prevent appreciable leakage between the ports 2, 3 even after the faces 11-14 have undergone considerable wear.

Since the areas where the control member 7 engages the housing are eccentric with reference tothe axis of the shaft 6, the pressures required to prevent leakage are lower than in known machines; this results in reduced wear and contributes to higher hydromechanical output of the machine. Another advantage is that the eccentric 5 imparts to the control member 7 a wobbling or translatory movement (not unlike a wedge or cam) whereby the stroke of the control member need not exceed between 0.5 and 1.5 times the diameter of a space 25. Those parts of the control member which are adjacent to the shaft 6 can or could perform a rotary movement celeration and deceleration characteristics because the mass of moving parts is relatively small.

The main purpose of the elastic sealing element 18 and of the springs 27 is to insure that the end faces 11, 13 remain in requisite sealing engagement with the faces l2, 14 when the shaft 6 is idle. Once the shaft rotates to cause the flow of fluid from the port 2 to the pressurizing spaces 25 and from such spaces to the port 3, or vice versa, pressure of fluid on the surfaces 20, 22 or 2l, 23 insures that the end faces of the rings 9, remain in satisfactory sealing engagement with the faces 14 and 12.

An advantage of the axially movable eccentric 5 shown in FIG. 2 is that its end face 33 prevents fluid from entering the bore for the shaft 6. The spring 30 insures that the bore for the shaft 6 is sealed from the inner chamber 15a when the shaft is idle. Once the shaft rotates and the pressure of fluid in the machine rises, the end face 33 is held in requisite sealing engagement with the face 14 mainly by fluid pressure. Another advantage of the structure shown in FIG. 2 is that the apertures for the passage of fluid from the chamber a to the port 2 or vice versa are provided between the rolling elements 35. This results in substantial savings in space (because the dimensions of the control member 7 can be reduced) and renders it possible to assemble the machine of a smaller number of parts because the inner ring 9 and the eccentric 5 can constitute components (races) of the antifriction bearing. Moreover, the arrangement shown in FIG. 2 enables the inner ring 9 to perform translatory rather than rotary movements. The improved control member can be used in hydrostatic machines with radially or axially movable pistons.

In the machine of FIG. 3, the shaft 6 extends through and beyond the eccentric 5 and cover l. A package of dished springs 37 is interposed between the eccentric S and a sleeve 38 to bias the end face 33 against the outer face 14. The compartment 19 is replaced by an annular groove 39 machined into the cover l and communicating with the port 2. Otherwise, the construction and mode of operation of the machine shown in FIG. 3 are identical with those of the machine shown in FIG. 2.

Referring to FIG. 4, there is shown a further fluid displacing machine which includes a stationary shaft 6 and a rotary unit including the housing (cover l and shell 4). The eccentric 5 is keyed to the fixed shaft 6, as at 40, and is not intended to move in axial direction of the shaft. The end faces of the eccentric are formed with substantially circular recesses 41, 42 for supporting ring-s 43, 44 which are biased by helical springs 47, 43 reacting against internal annular surfaces 45, 46 in the recesses 41, 42. The springs 47, 48 respectively bias the supporting rings 43, 44 against the faces 12 and 14. The nonoccupied portions 49, 50 of the recesses 4l, 42 constitute annular compartments through which the fluid flows when the machine is in operation. The ports 2, 3 are provided in the exposed end portion of the fixed shaft 6 and are respectively connected with the compartments 50, 49 by bores orchannels 57, 55 machined into the shaft 6. The compartment 49 further communicates with the chamber 26 by way of bores 54 machined into the cover l, concentric grooves 5l, 53 in the supporting ring 41, and axially parallel ducts 52 which connect the groove 5l with the groove 53. The compartment 50 connects the port 2 with the inner chamber 15a through the intermediary of one or more radial channels or bores 56 in the eccentric 5. When the parts ll, 4 and 7 rotate, the fluid is pumped from the port 2 into the spaces 25 and thereupon into the port 3 (or vice versa) in a manner which is readily apparent by looking at FIG. 4.

The springs 47, 48 in the recesses 4l, 42 serve to insure requisite sealing engagement between the supporting rings 43, 44 and the faces l2, 14 when the housing is idle. Once the housing rotates so that the pressure of fluid in the compartment 49 and/or S0 rises, the supporting rings are biased against the housing mainly by fluid pressure. The supporting rings 43, 44 surround sealing elements which are recessed into the periphery of the shaft 6. These supporting rings invariably prevent leakage of fluid along the shaft t5, not only outwardly toward the ports 2, 3 but also inwardly into the bore which accommodates the larger diameter portion of the shaft.

Without further analysis, the foregoing will so fully reveal the -gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of my contribution to the art.

What I claim as new and desire to be protected by Letters Patent is set forth in the appended claims:

1. In a hydrostatic machine of the character indicated, a combination comprising first and second units one of which is rotatable relative to the other in at least one direction, said first unit including a housing defining a first annular chamber and having a pair of parallel faces flanking the ends of said chamber, said second unit including a shaft mounted in said housing and an eccentric provided on said shaft in said chamber; and an annular control member rotatably surrounding said eccentric in said chamber and defining a second annular chamber sealed from said first chamber, said control member comprising a pair of rings movable axially of each other and each having an annular end face, and means for biasing said rings axially apart so that each of said end faces bears `against a different face of said housing, said other unit having fluid-admitting and discharging ports each communicating with a different one of said chambers and said housing further having at least one fluid-pressurizing space terminating in one of said faces thereof .and alternately communicating with said first and second chambers in response to relative movement between said housing and said control member on rotation of said one unit.

2. A combination as defined in claim 1, wherein the means for biasing said rings axially apart comprises mechanical biasing means.

3. A combination as defined in claim il, wherein all points of each of said end faces perform substantially translatory movements relative to the corresponding faces of said housing in response to rotation of said one unit, the extent of such translatory movement being equal to the radius of eccentricity of said eccentric.

4. A combination as defined in claim ll, wherein a portion of one of said rings is sealingly telescoped into the other ring.

5. A combination as defined in claim i, wherein said means for biasing said rings axially apart includes adjacent but spaced surfaces provided on said rings in at least one of said chambers so that the fluid in said one chamber acts against such surfaces to urge the rings against the corresponding faces of said housing.

6. A combination as defined in claim l, further comprising friction-reducing means interposed between said eccentric and said control member.

7. A combination as defined in claim 1, wherein said means for biasing said rings axially apart comprises spring means interposed between said rings.

8. A combination as defined in claim l, wherein said means for biasing said rings axially apart comprises an elastic sealing element interposed between said rings.

9. A combination as defined in claim 1, wherein said eccentric is movable axially of said shaft and further comprising means for biasing said eccentric axially against said one face of said housing.

l0. A combination as defined in claim 9, wherein the means for biasing said eccentric comprises a plunger located in the path of pressurized fluid.

1l. A combination as defined in claim l, further comprising antifriction bearing means interposed between said eccentric and said control member and defining at least one aperture connecting said second chamber with the respective port.

12. A combination as defined in claim 11, wherein said antifriction bearing means comprises an annulus of rolling elements surrounding said eccentric and defining a plurality of apertures.

13. A combination as defined in claim l1, wherein one ring of said control member constitutes a race of said antifriction bearing means.

14. A combination as defined in claim 1l, wherein said eccentric constitutes a race of said antifriction bearing means.

15. A combination as defined in claim l, wherein said eccentric is provided with a pair of concentric circular recesses each adjacent to one face of said housing and further comprising a pair of annular supporting members each received in one of said recesses and means for biasing said supporting members against the respective faces of said housing.

16. A combination as defined in claim l5, wherein said supporting members are sealingly received in the respective recesses and are biased by fluid against the respective faces of said housing.

17. A combination as defined in claim 15, wherein portions of said recesses constitute compartments each connecting one of said chambers with the respective port.

18. A combination as defined in claim 17, wherein one of said supporting members ismprovided with a pair of axially spaced concentric grooves and ducts connecting said grooves, said grooves and ducts connecting said first chamber with the respective compartment.

19, A combination as defined in claim 18, wherein said housing is provided with at least one passage connecting said grooves and said ducts with said first chamber, said one supporting member being adjacent to the other face of said housing, said ports being provided in said shaft and said shaft further having channel means connecting said one compartment with one of said ports.

20. A combination as defined in claim 17, wherein said ports are provided in said shaft and said shaft is further provided with channel means connecting one of said compartments with one of said ports, said eccentric having at least one bore connecting said one compartment with said second chamber. 

1. In a hydrostatic machine of the character indicated, a combination comprising first and second units one of which is rotatable relative to the other in at least one direction, said first unit including a housing defining a first annular chamber and having a pair of parallel faces flanking the ends of said chamber, said second unit including a shaft mounted in said housing and an eccentric provided on said shaft in said chamber; and an annular control member rotatably surrounding said eccentric in said chamber and defining a second annular chamber sealed from said first chamber, said control member comprising a pair of rings movable axially of each other and each having an annular end face, and means for biasing said rings axially apart so that each of said end faces bears against a different face of said housing, said other unit having fluid-admitting and discharging ports each communicating with a different one of said chambers and said housing further having at least one fluidpressurizing space terminating in one of said faces thereof and alternately communicating with said first and second chambers in response to relative movement between said housing anD said control member on rotation of said one unit.
 2. A combination as defined in claim 1, wherein the means for biasing said rings axially apart comprises mechanical biasing means.
 3. A combination as defined in claim 1, wherein all points of each of said end faces perform substantially translatory movements relative to the corresponding faces of said housing in response to rotation of said one unit, the extent of such translatory movement being equal to the radius of eccentricity of said eccentric.
 4. A combination as defined in claim 1, wherein a portion of one of said rings is sealingly telescoped into the other ring.
 5. A combination as defined in claim 1, wherein said means for biasing said rings axially apart includes adjacent but spaced surfaces provided on said rings in at least one of said chambers so that the fluid in said one chamber acts against such surfaces to urge the rings against the corresponding faces of said housing.
 6. A combination as defined in claim 1, further comprising friction-reducing means interposed between said eccentric and said control member.
 7. A combination as defined in claim 1, wherein said means for biasing said rings axially apart comprises spring means interposed between said rings.
 8. A combination as defined in claim 1, wherein said means for biasing said rings axially apart comprises an elastic sealing element interposed between said rings.
 9. A combination as defined in claim 1, wherein said eccentric is movable axially of said shaft and further comprising means for biasing said eccentric axially against said one face of said housing.
 10. A combination as defined in claim 9, wherein the means for biasing said eccentric comprises a plunger located in the path of pressurized fluid.
 11. A combination as defined in claim 1, further comprising antifriction bearing means interposed between said eccentric and said control member and defining at least one aperture connecting said second chamber with the respective port.
 12. A combination as defined in claim 11, wherein said antifriction bearing means comprises an annulus of rolling elements surrounding said eccentric and defining a plurality of apertures.
 13. A combination as defined in claim 11, wherein one ring of said control member constitutes a race of said antifriction bearing means.
 14. A combination as defined in claim 11, wherein said eccentric constitutes a race of said antifriction bearing means.
 15. A combination as defined in claim 1, wherein said eccentric is provided with a pair of concentric circular recesses each adjacent to one face of said housing and further comprising a pair of annular supporting members each received in one of said recesses and means for biasing said supporting members against the respective faces of said housing.
 16. A combination as defined in claim 15, wherein said supporting members are sealingly received in the respective recesses and are biased by fluid against the respective faces of said housing.
 17. A combination as defined in claim 15, wherein portions of said recesses constitute compartments each connecting one of said chambers with the respective port.
 18. A combination as defined in claim 17, wherein one of said supporting members is provided with a pair of axially spaced concentric grooves and ducts connecting said grooves, said grooves and ducts connecting said first chamber with the respective compartment.
 19. A combination as defined in claim 18, wherein said housing is provided with at least one passage connecting said grooves and said ducts with said first chamber, said one supporting member being adjacent to the other face of said housing, said ports being provided in said shaft and said shaft further having channel means connecting said one compartment with one of said ports.
 20. A combination as defined in claim 17, wherein said ports are provided in said shaft and said shaft is further provided with channel means connecting one Of said compartments with one of said ports, said eccentric having at least one bore connecting said one compartment with said second chamber. 